The visual refinement of any substrates, with which these substrates are given a metallic effect, has been known for a long time. Here, layers which are very different from each other are applied in a range of different sequences to the substrate, wherein said layers comprise at least one metallic layer. Essential requirements for substrates which are coated with a metallic layer are excellent corrosion resistance and a pleasing visual appearance, by means of which the coated substrates appear to be fully metallic or chrome-plated substrates, for example. Of particular importance are coated substrates in the automobile industry, for example, when manufacturing wheels or wheel rims, in particular light metal wheels or light metal wheel rims, for which a shiny, chrome appearance is required.
A method is known for example from the prior art with which light metal wheel rims are galvanically chrome-plated. With this method, a chrome layer, which is only several thousandths of a millimeter thick, is applied to a light metal wheel rim. In order to avoid reproducing all the unevennesses of the substrate surfaces, the wheel rims must therefore be ground, shiny polished and thoroughly prepared before the galvanic coating process. Otherwise, all pores, scratches and unevennesses can clearly be seen on the coated wheel rim. The grinding, shiny polishing and preparation of the substrate are very complex and require a great amount of work, regardless of the geometry of said substrate. Furthermore, the galvanic process is as such laborious in terms of work safety, and can harm the environment if it is not conducted correctly. As soon as the galvanically applied chrome surface is damaged, the known contact corrosion occurs. Under the influence of, e.g., rainwater or snow melt water, which generally contains dissolved road salt, an electric voltage series is created between the more precious part (in this case, chrome as the covering layer) and the less precious metal of the substrate (such as an aluminium or magnesium alloy). Here, the less precious metal disintegrates. As a result, e.g., a wheel rim can in an unfavourable case be severely damaged when inter-crystalline corrosion occurs, which can then lead to a critical effect on both the visual appearance and the stability levels of the wheel rim under dynamic load during use. It is furthermore of disadvantage with the galvanic chrome plating process that the galvanically applied chrome layer more frequently comprises other expansion coefficients than the substrate material which lies beneath it. As a result, tensions can occur which lead to fissures or even flaking.
Coating methods are also known from the prior art in which chrome is deposited onto a wheel rim by cathode spraying (sputtering) in a high vacuum. The method is conducted under high electric voltage. A light metal wheel rim which is coated using the sputtering method does not however usually have the same visual appearance as a galvanically chrome-coated wheel rim, i.e., rather than being metallic and shiny, it has the appearance of black chrome, and as a result has a darker surface which is by no means of the same value as a galvanically coated chrome surface. So-called “black chrome surfaces” are unacceptable, e.g., for all shiny sanitary items. Furthermore, a light metal wheel rim which has been manufactured using the sputter chrome coating method does not meet the test requirements which are specified as a minimum standard by the automobile industry, such as the grid cut test according to DIN EN ISO 2409, the salt spray test (copper chloride/acetic acid) according to DIN 50021-CASS(240h), the condensation water constant climate test according to DIN 50017 KK and the chemical resistance test according to VDA 621-412.
A method for the strong adhesive coating of a substrate is known from DE 102 10 269 A1, in order to give said substrate a metallic appearance, wherein initially a base coat layer is applied to the substrate and is dried, and the base coat layer is then treated with an inorganic bonding agent. A silver layer is then applied. Finally, the applied layers are coated with a protective lacquer. With the substrates coated using this method, an oxidation of the silver layer occurs relatively rapidly through the protective lacquer which is not completely sealed. This leads to a loss of adhesion of the silver layer from the substrate, and finally to a yellow discoloration.
In order to achieve sufficient corrosion protection of metal parts, coatings which contain chrome, also known as conversion layers, are frequently applied. Due to the light yellow iridescent effect of coatings of this type, the process is also referred to as yellow alodining. In contrast to anodically applied protective coatings, chromate conversion coatings no longer provide regular protection as soon as the surface is scratched. Alodined surfaces can be obtained by means of the immersion method or the injection/spray method. An example of the application of chromate protective layers can be found in U.S. Pat. No. 2,825,697 and U.S. Pat. No. 2,928,763. The application of a conventional conversion layer on a chrome base is also given, for example, in WO 2004/014646 A1.
A modified chromate coating is given in WO 01/51681 A2, according to which a suitable passivation solution must contain chromium(III) chloride and sodium nitrate.
In DE 197 02 566 C2, the method for shine coating motor vehicle parts is finally modified with the aid of a chromate layer to the extent that a very shiny layer made of a metal is applied in a vacuum to a powder lacquer layer present on the chromate layer using a magnetron. By means of this method, colour effects can be systematically created without the necessity of adding external pigments.
It is furthermore known from WO 01/51681 A2 and DE 602 00 458 T2 that metal layers can be made resistant to corrosion not only by means of treatment with a chromate which contains a passivation or conversion solution, but that for this purpose, metal phosphate coatings which do not dissolve easily such as coatings with a zinc or iron phosphate base can also be used.
For the chrome-free surface treatment, according to DE 103 32744 A1, an aqueous mixture containing an at least partially hydrolysed, fluorine-free silane and an at least partially hydrolysed silane which contains fluorine can also be used.
According to DE 602 00 458 T2, sufficient corrosion resistance can be achieved in that the corrosion protection coat contains a metallic zinc powder and at least one metal salt rust inhibitor, wherein this metal salt is based on magnesium, aluminium, calcium and barium, and has an average diameter size of no more than 1 μm. The metal in the metal salt must be more alkaline than zinc.
Good corrosion protection is achieved according to DE 100 49 005 A1 when the method stage of the treatment with a passivation agent occurs simultaneously with the application of a lubricant. The prerequisite for this is that the agent which contains lubricant does not essentially consist of titanium or/and zirconium and fluoride and a polymer. This new development essentially makes use of long-chain molecule residues which, as is known from surface active substances such as tensides, tend towards self-assembly. Accordingly, this technology is also known as SAM coating (Self Assembling Molecules).
A chrome-free surface coating of metals, which can be applied at high coating speeds, is according to DE 101 49 148 A1 based on an aqueous composition which contains an organic film creating agent which contains at least one polymer which does not dissolve easily or which is dispersed in water, with an acid value in the region of between 5 and 200, at least one inorganic connection in particle form with an average particle diameter in the range of between 0.005 and 0.3 μm and at least one lubricant, wherein the dried film which is applied comprises a layer thickness in the region of between 0.01 to 10 μm, a pendulum strength of between 50 and 180 s and a flexibility which prevents fissures longer than 2 mm from occurring when bent over conical pin in accordance with DIN ISO 6860. Synthetic resins based on acrylates, butadienes, ethyls, polyester, polyurethane, silicon polyesters, epoxy resins, phenol, styrol and urine formaldehyde are suitable for use as organic film creation agents.
U.S. Pat. No. 6,896,920 B2 discloses a multi-layer shiny coat with which initially, a polymer layer is to be applied to a metallic substrate surface. Then, this polymer coating is supplemented by a metal layer. An inorganic layer which prevents corrosion is then applied to this metal layer. The final, top layer of this multi-layer system is a transparent protective lacquer layer. Although it is identified as preventing corrosion, with the multi-layer substrates in accordance with U.S. Pat. No. 6,896,920 B2, with the CASS salt spray mist test, a corrosion-related change in the surface is determined after just 168 hours. The automobile industry regularly demands evidence of an unchanged surface even after 240 hours, however.